This invention relates to a catalyst for the oxidation of sulfur dioxide to sulfur trioxide.
Sulfuric acid is generally prepared on a commercial scale by the gas phase oxidation of sulfur dioxide to sulfur trioxide followed by absorption of the sulfur trioxide in an aqueous medium. The oxidation of sulfur dioxide to sulfur trioxide is generally carried out over an absorptive carrier containing vanadium.
Various processes for preparing supported sulfuric acid catalysts have been described in the prior art.
Thus, U.S. Pat. No. Re. 19,282 discloses the addition of potassium or sodium hydrate, carbonate, sulfate or nitrate together with a vanadium compound to a very finely divided carrier. After the catalyst is molded or pressed into the desired shape, it is treated with gases containing sulfur dioxide, such as burner gases.
U.S. Pat. No. 1,862,825 describes spraying a natural diatomaceous earth aggregrate with a solution of sodium vanadate, drying and heating, preferably in an atmosphere of sulfur dioxide. However, sodium vanadate is generally not used in commercial operations since the potassium salt has been found to be more active.
In U.S. Pat. No. 3,216,953, a vanadium-potassium-silica catalyst is described. This catalyst is made by mixing aqueous potassium hydroxide with vanadium oxide and dilute sulfuric acid in such quantity as to maintain an alkaline solution, mixing the solution with ammonium hydroxide and adding the resulting mixture to silica particles. The ammonium hydroxide is required in order to solubilize the potassium and vanadium compounds, for example, potassium vanadate and potassium sulfate, which are formed in the alkaline reaction medium.
The use of vanadyl sulfate and potassium vanadyl sulfate in multi-step processes for preparing sulfuric acid catalysts has also been disclosed. For example, U.S. Pat. No. Re. 18,380 describes passing sulfur dioxide through an aqueous suspension of vanadium oxide in sulfuric acid to provide blue vanadyl sulfate. Part of this blue vanadyl sulfate solution is treated with potassium hydroxide to provide brown potassium vanadite. Then the brown potassium vanadite is mixed with diatomaceous earth and the remainder of the blue vanadyl sulfate solution added. The resulting support is then treated with gases containing sulfur dioxide gas prior to use as a catalyst. Thus, both vanadyl sulfate and potassium vanadite are required to provide the desired catalyst.
U.S. Pat. No. 1,696,546 describes base exchanging silicate catalysts containing both catalytic cations and anions. One example involved passing sulfur dioxide through an aqueous mixture of vanadium oxide and potassium hydroxide to provide a precipitate of potassium vanadyl sulfate which is then dissolved by means of potassium hydroxide. The resulting brown solution is used together with a solution of chrome alum in potassium hydroxide and a mixture of aqueous potassium silicate and a carrier to provide the desired catalyst.
Unsupported sulfuric acid catalysts, such as the mixture of a vanadium compound in a molten alkali metal sulfate disclosed in U.S. Pat. No. 2,381,908, have also been described in the literature. However, such systems have not found wide-spread commercial use.
The above described catalysts are all subject to certain drawbacks which make them unattractive for use in commercial applications. Thus, some of them require several solutions in treating the carrier, while others require a subsequent treatment of the catalysts with gases containing sulfur dioxide prior to use, in order to produce hard substances that withstand stress during transportation to users and during loading in commercial converters.
Now it has been found in accordance with this invention that a supported sulfuric acid catalyst can be made by wetting diatomaceous earth with a solution of potassium sulfate and vanadyl sulfate made by passing sulfur dioxide gas into an aqueous mixture of vanadium oxide, sulfuric acid and a potassium salt compatible with the reducing reaction in situ. The diatomaceous earth is then dried and calcined to provide a catalyst which is suitable for use directly in a sulfuric acid process without requiring additional sulfating, thus permitting catalyst production independent of the availability of gas streams from a converter. Furthermore, the use of only one solution to treat the diatomaceous earth presents an additional commercial advantage.